UvA-DARE (Digital Academic Repository) Luminescent Lanthanide Complexes: Visible Light Sensitised Red and Near- infrared Luminescence. Werts, M.H.V. Publication date 2000 Link to publication Citation for published version (APA): Werts, M. H. V. (2000). Luminescent Lanthanide Complexes: Visible Light Sensitised Red and Near-infrared Luminescence. General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl) Download date:08 Oct 2021 Chapterr 7 Interactionss between push-pulll chromophores andd lanthanide (3-diketonates Fromm the near-infrared luminescent complexes described in the previous three chapters we returnn to the visibly emitting europium(HI) complexes, at least during the first part of this chapter.. Accidentally a complex was discovered which produces photosensitised Eu3+ luminescencee upon excitation with visible light. This chapter starts out with a detailed studyy of this complex and similar ones. The concepts extracted from this study are then appliedd to near-infrared luminescent lanthanide complexes and to the design and synthesis off complexes more suited for practical application. 7.11 A surprising result Whenn colourless solutions of Michler's ketone (4,4'-bis-(M#-dimethylamino)-benzophe- none,, MK) and EuFOD (europium tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyloctane-3,5- dione))) in benzene (both 1 mM) are mixed, a yellow colour develops instantaneously. Moreover,, a red glow emerges from the solution under daylight illumination. The emission spectrumm (Figure 7-2) demonstrates that this red glow is Eu3+ luminescence: the sharp peakss are characteristic of lanthanide ion emission, Eu usually having its most intense emissionn around 615 nm. The corresponding excitation spectrum is in accordance with the observationn that this luminescence can be excited by visible light. It extends well beyond 4500 nm (Xmax 414 nm). The quantum yield was found to be 0.17 in aerated solution and 0.200 after deoxygenation by four freeze-pump-thaw cycles (excitation at 420 nm, using quininee bisulfate in 1M H2S04 as a reference^). Thesee observations were made during experiments aimed at the observation of intermo- lecularr energy transfer from organic sensitisers to lanthanide complexes and are quite sur- prising.. The colour change indicates a ground-state interaction between the components. Moreover,, the product of this interaction seems to defy the rule-of-thumb that only chromophoress with their lowest singlet-singlet transitions in the UV can efficiently sensi- tisee Eu luminescence. ** Especially in Eu p-diketonates, this so-called hypersensitive Dt —» F2 transition is usually very intense.^1'' See also Chapter 3. 1022 CHAPTER 7 .:EU* * Michler'ss ketone, Mk EuFOD Figuree 7-1. Structural formulae of Michler's ketone and EuFOD. Itt was hypothesised that Michler's ketone and EuFOD form a ground-state complex by interactionn of the electron rich carbonyl group with the positively charged Eu + ion. Such interactionss are already well known from the use of lanthanide diketonates as NMR shift reagentss J3' 4^ and in fact coordination of certain aliphatic substrates to EuFOD has been foundd to enhance its luminescence quantum yield. Never before, however, has it been reportedd that complexation of chromophores to lanthanide P-diketonates yields efficiently luminescentt Eu3+ complexes that absorb visible light. Thee new absorption band is probably due to a bathochromic shift of the first singlet-sin- glett transition of Michler's ketone occurring upon complexation. This n-n* transition pos- sessess charge-transfer character^ ' in the process of excitation, electron density is moved towardss the carbonyl group and therefore it is quite likely that the transition energy is largelyy affected by the presence of a lanthanide ion. Since for a lanthanide complex to be luminescent,, the triplet state of the antenna chromophore needs to lie at higher energies thann the luminescent state of the ion, the singlet-triplet gap of the complexed MK must be small.. It is known that free MK has such a small gapJ7^ Excitation n Emission n in in c c S S > > a> > DC C _A—J J —i— — 400 0 500 0 600 0 700 0 A/nm m Figuree 7-2. Corrected luminescence excitation (Xem=612 nm) and emission (Xexc=450 nm) spectraa of a solution of 10'5 M Michler's ketone and 10"4 M EuFOD in benzene. PUSH-PULLL CHROMOPHORES AS SENSITISERS 103 Inn the next sections we will confirm these hypotheses, and describe in more detail the structuree and the photophysics of MK-EuFOD and related complexes. 7.22 Structure of the complex of MK and EuFOD Uponn closer inspection, the coordination of MK to lanthanide (3-diketonates was found to occurr only in non-coordinating solvents. Also, water molecules can compete with MK for freee coordination sites on the lanthanide ion. Therefore, all reagents and solvents were driedd before use. MK was purified according to a literature procedure^ ] 7.2.11 Spectrophotometric titration Thee formation of a complex between MK and EuFOD in benzene can be monitored using UV-Viss absorption spectroscopy. Figure 7-3 shows a titration experiment, in which small amountss of a EuFOD stock solution are added to a solution of MK. It clearly shows 1:1 complexx formation of MK with EuFOD. The absorption band of the complex resides at longerr wavelengths, where MK and EuFOD themselves do not absorb. Analysis of the titrationn data (see Section 7.3.1) yields the formation constant for the MK-EuFOD com- plexx in benzene, log K = 4.85. The extinction coefficient at 414 nm, the absorption maxi- mum,, was found to be 3.04 x 104 M"1 cm"1. €o €o -O O < < Figuree 7-3. UV/Vis titration of a solution of Michler's ketone in benzene with EuFOD. Too 2.5 mL off a 10'5 M solution of MK in benzene, a EuFOD solution (5 x 10"4 M in benzene) iss added in steps of 50 uL. The inset shows formation of the complex by means of itss absorption at 414 nm as a function of the amount of EuFOD solution added. The solidd line is the theoretical curve (Equation 7-2) for a 1:1 complex with log K = 4.85 4 1 1 andd emax = 3.04 x 10 M' cm" . The dilution has been taken into account. 1044 CHAPTER 7 7.2.22 Other p-diketonates: change of ligands and of central ion Thee effect of variation of the (3-diketonate was explored. In benzene, the maximum of the absorptionn band varies from 398 nm for MK-Eu(dpm)3 (dpm = 2,2,6,6-tetramethylhep- tane-3,5-dionate)) via 414 nm for MK-EuFOD to 430 nm for MK-Eu(hfa)3 (hfa = l,l,l,5,5,5-hexafluoro-2,4-pentanedionate).. We tentatively attribute this to a variation of thee electrostatic interaction between the MK chromophore and the lanthanide ion in the complexes,, related to changes in chromophore-ion distances (see Chapter 7.2.3). Unlike MK-Eu(dpm)33 and MK-EuFOD (both having 0.20 quantum yield in deoxygenated ben- zenee at room temperature), the sensitised luminescence quantum yield of MK-Eu(hfa)3 is veryy low (6.2 x 10"5 under the same conditions). This might be a result of too low a triplet energyy of the antenna chromophore and we will discuss that later. Thee same bathochromic shift of the MK absorption as induced by EuFOD is also observedd with other lanthanides. Addition of either YbFOD, ErFOD, GdFOD or PrFOD to MKK in benzene produces the same 414 nm absorption band in each case. This reflects the electrostaticc nature of the interaction: all lanthanide ions carry a 3+ charge and the distance betweenn the ion and the MK should be roughly equal in all complexes, since the structures off all the FOD chelates are expected to be practically the same. They will therefore have thee same effect on the electronic energy levels of the chromophore. Obviously, the red gloww is only observed with EuFOD. Althoughh the induced "optical shifts" are the same for different LnFODs, the shifts of thee MK *H nuclear magnetic resonances induced by these complexes are entirely different, whichh clearly shows that the effects have different origins (electrostatic vs. magnetic). For example,, EuFOD causes a downfield shift whereas PrFOD shifts the resonances upfield. Thee protons closest to the carbonyl group are affected most, and the protons on the dimeth- ylaminogroupss are relatively unaffected. This supports our view that the MK interacts with thee lanthanide diketonate through its carbonyl group. 7.2.33 Semi-empirical (SMLC/AM1) quantum chemical investigation Resultss from molecular modeling also point in the direction of complex formation by meanss of the MK's carbonyl group. We implemented the SMLC (Sparkle Model for Lan- thanidee Complexes) modification t9,10^ to the AMI method in the MOPAC 6.0 computer programm which allows for the calculation of the structures of lanthanide complexes.